Southern California Edison said 20,000 customers in Los Angeles, Orange, Ventura, Riverside and San Bernardino counties had no electricity Monday.
About 9,000 more customers in scattered parts of Los Angeles also were without power, Los Angeles Department of Water and Power spokeswoman MaryAnne Piersen said.
Temperatures Monday hit 99 degrees in downtown Los Angeles and reached 111 degrees in the Woodland Hills area of the San Fernando Valley, according to the National Weather Service.
Firefighters found an elderly couple in their apartment in a San Fernando Valley neighborhood where 106 degree were reported. The couple did not have air conditioning, authorities said.
San Diego Gas and Electric Co., which serves San Diego County and southern Orange County, declared a power emergency and began preparing for potential rolling blackouts as demand hit a record.
About 30,000 of its customers experienced outages Monday, but electricity was restored to 22,000 of them by the afternoon, spokesman Peter Hidalgo said.
"We need immediate energy conservation, or else there will be rolling blackouts," Hidalgo said.
Lightning strikes on grid equipment due to scattered desert thunderstorms added to the strain.
The California Independent System Operator, which oversees the state's power grid, said no major shortages were expected. But it was urging customers to conserve electricity by setting air conditioning thermostats higher and waiting to use major appliances until after dark.
Relief is in on the way, reported CBS News correspondent Bill Whitaker. Forecasters predict a cool front will drop temperatures 15 degrees by the end of the week.